Warming accelerates development but alters synchrony in a season‐dependent manner in a damselfly

Abstract Developmental synchrony—the coordinated timing of life stage transitions—is a key life‐history trait in temperate insects. Based on theory and previous studies, we expected that warming and late‐season photoperiods would shorten development time and increase synchrony, and that their effects would interact. We investigated how environmental variables shape larval development time and synchrony in a central‐core population of the univoltine damselfly Lestes sponsa . Larvae were reared to the penultimate instar before emergence under four independent treatments combining early‐ or late‐season photoperiods with ambient or elevated temperatures, using seasonally fluctuating spring or summer conditions to mimic natural environments. As expected, development time decreased under warming and late‐season photoperiods. However, synchrony patterns did not fully align with predictions: warming reduced synchrony under early‐season conditions, and late‐season treatments showed lower synchrony at the individual level but not at the family level. These results suggest that photoperiod and temperature can shape synchrony, but not uniformly across photoperiod‐temperature combinations or between individual and family levels. In central‐core populations, a longer developmental window may reduce the need for tight synchrony and instead favour more variable strategies such as bet‐hedging.